1. Field Of the Invention
The subject invention relates to resilient electrically conductive terminal assemblies for use in high density circuit applications, such as connecting high density memory modules to a circuit board.
2. Description Of the Prior Art
Memory modules include a generally planar rectangular ceramic body with an integrated circuit chip centrally therein. Electrically conductive leads extended from the chip to the periphery of the ceramic body. Until recently, memory modules were substantially as shown in FIG. 1. More particularly, the prior art memory module 10 of FIG. 1 has electrically conductive pins 12 extending outwardly from the ceramic body. The pins 12 are generally L-shaped, and include a first leg projecting from a side edge of the ceramic body generally parallel to the plane of the circuit board 14, and a second leg projecting downwardly approximately orthogonally to the rectangular chip. The pins 12 project through apertures 16 in the circuit board and are soldered to electrically conductive paths printed or otherwise disposed on the circuit board 14. The soldered connections between the pins 16 and the electrically conductive paths on the circuit board 14 are visible and accessible. Thus, the prior art assembly shown in FIG. 1 enables the quality of the soldered connections to be optically assessed.
The prior art memory module typically is the most expensive element on the board. It is not uncommon for a prior art memory module to cost between $50.00 and $100.00. The entire board, prior to mounting the memory module thereto also might cost $50.00-$100.00. The completed board invariably is tested prior to final installation into a computer or other piece of electronic equipment. If possible, any observed defect would be corrected, rather than discarding the entire board. For example, if a memory module was found the be defective, the accessible soldered connections might be desoldered. The defective memory module would then be discarded and a new memory module would be soldered to the board. If the board was found to be defective, the memory module could be desoldered and used on another board.
Memory modules have steadily become more complex and sophisticated without corresponding increases in size. Initially, the greater complexity led to more leads extending from the side edges and a corresponding increase in the number of apertures in the circuit board. However, the increase in the number of apertures was found to cause local weaknesses in the circuit board. In response to these problems surface mount memory modules were developed for mounting directly to the surface of a circuit board without a dense array of through holes. With reference to FIG. 2, the prior art surface mount memory module 10a has either short leads 12a projecting from side edges or contact pads along side edges that are soldered to contact pads 16a on the surface of the circuit board 14a. The prior art surface mount memory module 10a enables somewhat greater circuit densities without weakening the board 14a. The prior art surface mount memory module 10a still enables optical inspection of soldered connections and permits desoldering when necessary.
Memory modules have continued to increase in complexity without corresponding increases in size. The greater circuit densities enabled by the more complicated memory modules could not readily be accommodated along the peripheral edges of the memory module. Furthermore, connections along peripheral edges of the memory module require a bigger circuit "footprint" which offsets the miniaturization being achieved within the memory module. As a result, memory modules were developed with conductive paths leading to the bottom surface for mating with a corresponding array of conductive paths on the circuit board. An example of such a prior art high density memory module is illustrated schematically in FIGS. 3 and 4. In particular, the memory module 10b includes a plurality of conductive dots 12b on the bottom face thereof. The circuit board 14b includes a corresponding array of conductive pads 16b. Current technology permits the dots 12b and pads 16b to be disposed at center-to-center spacings, as indicated by dimension "a" in FIG. 3 of approximately 0.050 inch, and further miniaturization is possible. The prior art memory module 10b is accurately positioned such that the conductive dots 12b contact the conductive pads 16b. The circuit board is then subjected to wave soldering, or other known soldering techniques, to permanently connect the memory module 10b to the circuit board 14b as shown in FIG. 4. However, in contrast to the prior art embodiments depicted in FIGS. 1 and 2, the soldered connections in FIG. 4 are not visible and cannot be optically checked. Furthermore, the soldered connections in FIG. 4 are not accessible and hence the memory module 10b cannot readily be removed if a defect is subsequently observed in either the memory module 10b or the circuit board 14b. The more complex and sophisticated memory modules shown in FIGS. 3 and 4 often are significantly more costly than the prior art memory modules depicted in FIGS. 1 and 2. It is not uncommon for a memory module to cost more than $100.00, and some cost as much as $500.00. Additionally, the circuit boards for these sophisticated memory modules also are more complex, and hence more costly than their simpler predecessors. The difficulties of desoldering the inexcessible connections shown in FIG. 4 may force a computer manufacturer to discard both a memory module and a circuit board. Often either the discarded memory module or the discarded board will be perfectly functional. In some instances both the discarded memory module and the discarded board will be functional, and the defect will merely exist in a soldered connection between the two. The component manufacture would prefer not to discard a perfectly good memory module costing several hundred dollars, nor a good circuit board costing in excess of $100.00.
In view of these problems, the prior art has developed a high density memory module socket assembly as shown schematically in FIGS. 5 and 6. The prior art memory module socket assembly uses the circuit board 14b and the memory module 10b described and illustrated above. However, the prior art socket assembly further includes a base 18 having an array of apertures 20 extending therethrough and registered with the contact pads 16b on the circuit board 14b. The apertures 20 are filled with a jumbled array of very thin conductive wire 22 resembling a small steel wool pad. A jumbled wire array 22 is urged into each aperture 20, and is dimensioned to extend beyond the opposed surfaces of the prior art base 18. Thus, the wire 22 in the aperture 20 will engage a conductive pad 16b on the circuit board 14b and will engage a corresponding conductive pad 12b on the memory module 10b to provide electrical connection therebetween. Solder is entirely avoided, and mechanical means are used to hold the memory module 10b and the prior art base 18 in proper registration on the circuit board 14b. The memory module 10b can be removed and replaced or repositioned for any reason, such as an observed defect in either the memory module 10b or the circuit board 14b.
The connector assembly shown in FIGS. 5 and 6 overcome several of the disadvantages described with respect to the soldered connection depicted in FIGS. 3 and 4. However, the prior art connector assembly shown in FIGS. 5 and 6 also has drawbacks. One such drawback is cost. Prior art connectors, as shown in FIGS. 5 and 6, often cost between nine cents and fifteen cents per connection. Thus, a memory module with 500 conductive pads would have a connector costing $45.00-$75.00. Second, it is difficult to ensure that the jumbled array of wire 22 will exert the specified pressures against both the walls of the aperture 20 through the base 18 and on the conductive pads 12b and 16b on the memory module and board respectively. The entire jumbled array of wire 22 will fall out of the aperture 20 if the engagement forces are too low. Similarly, poor electrical connection will be achieved if the contact forces between the jumbled array of wire and the memory module for the board are too low. Furthermore, the jumbled array of wire 22 is not well suited to making plural make and break connections. Thus, if a defect in the memory module is observed or if it is desired to merely change to a different memory module, the jumbled array of wire 22 may not resiliently return a sufficient amount to make a good second connection.
In view of the above, it is an object of the subject invention to provide a connector for a high density memory module.
It is another object of the subject invention to provide a memory module connector that enables repeated connection and disconnection of high density memory modules therefrom.
It is a further object of the subject invention to provide an electrically conducted terminal assembly for connecting the contact pad of a memory module to the contact pad of a circuit board.